<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8" />
		<title>Document</title>
		<style>
			#test{
				width: 400px;
				height: 400px;
				background-color: orange;
				font-size: 20px;
			}
			html,body{
				font-size: 10px;
			}
			#demo{
				width: 10rem;
				height: 10rem;
				background-color: gray;
			}
		</style>

	</head>
	<body>
		<div id="test">
			<div id="demo"></div>
		</div>
	</body>
</html>